/Engine/Source/Editor/Experimental/EditorInteractiveToolsFramework/Public/EdModeInteractiveToolsContext.h
Utility function to convert viewport x/y from mouse events (and others?) into scene ray. Copy-pasted from other Editor code, seems kind of expensive?
static FRay GetRayFromMousePos ( [FEditorViewportClient](API\Editor\UnrealEd\FEditorViewportClient) * ViewportClient, [FViewport](API\Runtime\Engine\FViewport) * Viewport, int MouseX, int MouseY )